Propagation Of Electromagnetic Pulses Research Articles

Hybrid Ray–FDTD Moving Window Approach to Pulse Propagation

A moving window finite difference time domain method is developed to simulate the propagation of electromagnetic pulses over large distances. Both Eulerian and Lagrangian approaches to solving Maxwell's equations in a moving window are obtained and contrasted. The Lagrangian approach is shown to be superior for electromagnetic pulse propagation; it is demonstrated that dispersion-free numerical propagation can be achieved with the Lagrangian approach. Examples of propagation in homogeneous and inhomogeneous media, and scattering from a interface between two media are considered. The scattering results are achieved with a window splitting approach in which the original incident pulse window is frozen at the interface and new windows are generated after the interface interactions occur that move with the reflected and transmitted pulses. The simulation results are shown to be accurate and physically appealing.

Electrical generation of terahertz electromagnetic pulses by hot-electrons in quantum wells

We present a theoretical study on the electrical generation of terahertz (THz) electromagnetic pulses by hot-electrons in AlGaAs/GaAs-based two-dimensional semiconductor systems (2DSSs). The intensity of THz blackbody radiation has been calculated as a function of photon frequency and electron temperature. We consider that the electrons are heated by pulsed electric fields and that the hot-electron interactions with electromagnetic fields are mediated by electron--phonon scattering.

Theory for the propagation of short electromagnetic pulses

A theory is presented for the free-space propagation of short pulses of any duration, including sub-cycle or unipolar pulses. From Maxwell's equations, we provide an exact solution of the vector potential and the scalar potential which result for an electric dipole created stepwise at t = 0 with a corresponding Dirac-delta in source current density. This idealized unipulse provides valuable insight as to the separation of 1 r propagating terms from those of higher order arising from the scalar potential. Secondly, an exact solution is presented for the radiation field in temporal form which arises from an assertion of an input aperture distribution. Since the temporal assertion may lead to propagation terms of 1 r dependence and others of higher order, we illustrate how to make an informed choice. A finite-difference-time-domain computer study of four selected sub-cycle pulses is used to illustrate the method and to verify our prediction that free-space propagation is a linear filter of temporal frequency varying as [ i2πv (rc)] exp( − i2πvr c ) with a null at zero frequency.

Ultrafast optoelectronic switches based on high-T/sub c/ superconductors

We present measurements of the ultrafast reflectivity change of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin films upon femtosecond laser excitation. The response is attributed to the breaking of Cooper pairs and the subsequent recombination of quasiparticles. In a current-biased superconducting bridge, these transient processes lead to the generation of picosecond electromagnetic pulses, which can be directly detected by measuring the emitted radiation from the bridge in a quasi-optical setup. We show that the emitted pulse can be calculated from the all-optical measurements.

Time reversal array focal zone predictions using simulations in the time domain

The use of finite-difference time-domain (FDTD) computer simulations for wave equation calculations has deep roots in the electromagnetic pulse propagation literature, where many of the modern techniques were developed. Results are presented from high-resolution simulations of the linear acoustic wave equation in inhomogeneous media with absorbing boundary conditions. The results address the focusing behavior of sparse ultrasonic arrays under various propagation conditions. Focal spot attributes and size are investigated in the presence of inhomogeneities, time-varying media, and multiple scatterers. Comparison with known benchmarks, such as the diffraction limit, show that useful results can be obtained using FDTD calculations in medical applications, for example, where the frequencies and scales allow full-sized simulations in major organs. [Work supported by the Office of Naval Research.]

Propagation of electromagnetic pulses in an infinite Drude medium

We analyze the propagation of an harmonic pulse launched at t=0 with some boundary condition at z=0 in an infinite Drude medium. This initial-boundary value problem is studied analytically in the low- and high-frequency limits by use of the Laplace transformation and subsequent application of asymptotic methods for the evaluation of the requisite integrals. We discuss the qualitative features of these approximations.

Ultra‐wideband electromagnetic pulse propagation in a homogeneous, cold plasma

In this paper, we investigate the propagation of an ultra‐wideband electromagnetic pulse in a homogeneous, cold plasma which is used to represent a simplified model of the atmosphere. The standard procedure for the computation of the corresponding transient field involves the application of a fast Fourier transform (FFT) to a well‐known, analytical, frequency‐domain solution. However, because of the long tails in both the time and frequency domains, a large number of sample points are required to compute the transient response using this FFT approach. In this paper, we introduce a new asymptotic extraction technique which dramatically reduces the number of sample points required by the FFT. First, we review the recently derived closed‐form expression for a double‐exponential pulse propagating in a homogeneous, collisionless, cold plasma. Since the high‐frequency behavior does not depend on the electron collision frequency, an analytical frequency‐domain expression, which is similar in form to the one encountered for the collisionless, cold plasma and encompasses this high‐frequency behavior, can be subtracted from the exact expression for the plasma with a nonzero collision frequency. The extracted term is evaluated analytically. The remaining expression, which can be transformed to the time domain with a FFT, requires only a modest number of sample points. This dramatically improves the numerical efficiency of the approach. We find that the extracted analytical term provides a very good approximation for the early‐time behavior of the transient pulse.

Generation of high-power ultrawideband electromagnetic pulses in a system with a coaxial tem horn

A coaxial TEM horn was designed on the basis of results from nonstationary computer modeling using code KARAT. With its high dielectric strength, this antenna is capable of radiating high-power ultrawideband nanosecond pulses. The pulse source used was a compact generator built around a coaxial forming line with a built-in Tesla transformer, which shapes pulses up to 1 GW high at repetition frequencies up to 1 kHz. The amplitude of the pulses on a matched load was 20 kV at a duration of 4 nsec. Returns of ultrawideband signals from objects with simple geometric shapes were studied in laboratory experiments using this radiator.

Asymptotics and energy estimates for electromagnetic pulses in dispersive media

We examine electromagnetic pulse propagation in anomalously dispersive media, using the Debye model as an example. Short-pulse, long-pulse, short-time, and long-time approximations and amplitude rate-of-decay estimates are derived with asymptotic methods. We also study the following problem: If we know only the peak amplitude and the energy density of an incident pulse, what can be said about the amplitude of the propagated pulse? We provide tight upper and lower bounds for the propagated amplitude, which may be useful in controlling the electromagnetic interference or the damage produced in dispersive media. We explain a factor-of-nine effect in the speed of pulses in a Debye model for water, which seems to have been previously unnoticed, and we also explain some observations from experimental studies of pulse propagation in biological materials. Finally, we propose some guidelines for sample size in transmission time-domain spectroscopy studies of dielectrics. Most of our results are easily extended to multiple space dimensions.

A Comment on "Propagation of ultrawide-band electromagnetic pulses through dispersive media" [and reply

In the above paper (see ibid., vol.37, p.192, 1995) the authors went to great lengths to derive an explicit solution in terms of incomplete Lipschitz-Hankel integrals (ILHI's) to the propagation problem of a double exponential pulse in a homogeneous medium. Technically, He has no doubt on the correctness of their ILHI representation. However, He doubts their claim about the computational efficiency of the ILHI representation. Dvorak and Dudley reply to the Comment.

Transient electromagnetic wave propagation in transverse periodic media

In this paper, a time-domain technique is used to analyze the wave propagation of electromagnetic pulses in media that are periodic in one spatial direction, perpendicular to the direction of propagation. The wave equation is reduced to a system of one-dimensional first order time dependent hyperbolic equations by a Fourier expansion in the transverse spatial direction and by utilizing a wave splitting technique. The method is especially useful for the propagation of broad pulses.

Subpicosecond electromagnetic pulse generation from high-Tc superconducting thin films by femtosecond laser pulse excitation

Ultrashort electromagnetic pulses with a halfwidth of 0.45 psec have been radiated from current biased superconducting YBCO films by exciting femtosecond laser pulses. Characteristics of the radiation are studied in detail.

Fast Fourier transform computational method for the propagation of electromagnetic pulses through layered dielectric media.

A computational method is developed for solving the wave equation for the propagation of a pulse through layered dielectric media. The method is based on an ansatz for evaluating fields in each layer after a temporal interval dt using a Fourier method. The resulting spatial fields represent the solution to the wave equation provided dt is small enough that many temporal intervals are required for the passage of the pulse through a boundary. \textcopyright{} 1996 The American Physical Society.

Abnormal velocity of soliton-type pulses in a nonlinear three-level medium with population inversion.

We present an analytical solution of the Maxwell-Schr\"odinger set of equations describing propagation of a two-component short electromagnetic pulse through a medium consisting of atoms with a @sV-type level scheme with an initially inverted population. This solution demonstrates solitonlike waves traveling at a velocity exceeding the vacuum speed of light c (or even a negative velocity) until they reach the forward (or, respectively, trailing) pulse front. The front propagation velocity recognized as information transmission velocity is explicitly found to be equal to c.

Generation and detection of terahertz electromagnetic pulses from semiconductors with femtosecond optics

The basic concepts and preliminary applications of THz photoconductive radiation from semiconductors are presented. We have studied THz radiation from different metallic electrodes with different geometries. We also report the experimental results from unbiased semiconductors with different surface and interfacial conditions, including chemically etched surfaces and metal/semiconductor interfaces.

Critical damping of wake-field oscillations in a dispersive and dissipative medium: Influence of nonlinearity

Wake-field oscillations which appear in a dispersive and dissipative medium (plasma) are found to disappear at a critical level of the dissipative effects, such as collisional damping in a plasma. Linear analysis, carried out for the purpose of application of Green's function techniques to the propagation of ultrashort electromagnetic pulses, shows that no wake-oscillations occur for v2/ωp2 ⩾ 0.8, v being the collision frequency and ωp the plasma frequency. For the domain where v2/ωp2 ≈ 0.8 i.e. when the medium-dependent linear term of a renormalized form of the field equation is negligible, the influence of non-linearity should become relatively important, particularly for high fields.Particular nonlinear solutions are given for a nonlinearity of the form βUp, where β and p are constants, and U is the renormalized field variable, assuming that the corresponding linear term can be neglected due to compensation between the dispersive and dissipative effects.It is found that, for p = 3, nonlinear oscillations occur when the coefficient of the nonlinear term has the same sign as the corresponding coefficient (ωp2 - (5/4)v2) of the linear term would have for ωp2 > (5/4)v2. The nonlinear oscillations are described by an elliptic integral of the first kind. The frequency of nonlinear oscillations is found to become proportional to √β and to the amplitude of the nonlinear oscillations.For the opposite sign of the nonlinear term the dissipation dominated solutions correspond to nonoscillatory decaying fields, expressed in terms of nonlinearly transformed variables in space and time. It is furthermore, found that for (5/4)v2 > ωp2 and provided the nonlinear and linear terms have opposite signs, soliton-type solutions exist for p = 3.

The wave hierarchy for propagation in relaxing dielectrics

We consider the propagation of arbitrary electromagnetic pulses in anomalously dispersive dielectrics characterized by M relaxation processes. A partial differential equation for the electric field in the dielectric is derived and analyzed. This single equation describes a hierarchy of M + 1 wave types, each type characterized by an attenuation coefficient and a wave speed. Our analysis identifies a “skin-depth” where the pulse response is described by a telegrapher's equation with smoothing terms, travels with the wavefront speed, and decays exponentially. Past this shallow depth we show that the pulse response is described by a weakly dispersive advection-diffusion equation, travels with the sub-characteristic advection speed equal to the zero-frequency phase velocity in the dielectric, and decays algebraically. The analysis is verified with a numerical simulation. The relevance of our results to the development of numerical methods for such problems is discussed.

Correlations in the coherent transient electron‐hole system

AbstractIn the frequency domain close to the intrinsic absorption threshold the dynamic polarization, accompanying propagation of a strong electromagnetic pulse, is a dense cloud of electron‐hole pairs, free or bound to excitons or both. In order to investigate the structure and properties of this system in a wide range of pumping field strengths and frequencies, including both the exciton resonance and continuum edge, two simplified models are introduced and discussed, both substituting an effective short‐range interaction instead of the usual Coulomb potential. To go beyond the mean‐field approximation (MFA) the correlation effects are accounted for phenomenologically in this interaction. Results for both ‐ (quasi‐) stationary regime and transient response ‐ are presented. Possible inelastic relaxation mechanisms and kinetics are discussed. For not too fast (in the “excitonic” time scale) varying pumping field strengths the Boltzmann equation for renormalized Galitskii‐Elesin quasi‐particles is derived.

The electromagnetic autowaves in a weakly conductive easy-axis ferromagnet

Propagation of picosecond electromagnetic video pulses along the easy magnetization axis of a non-equilibrium weakly conductive ferromagnet located in an external magnetic field parallel to this axis has been studied. For magnetic field components, solutions in the form of two types of autowave dissipative structure have been obtained. It is shown that these autowave dissipative structures can be formed under the restrictions on the magnetic anisotropy parameter and the Zeeman splitting frequency.

Nonlinear propagation of electromagnetic pulses in two-level media under strong coupling

We consider the problem of the propagation of short-duration light pulses in a two-level medium in the regime of strong interaction, near the resonance, with no restriction on the strength of the coupling between field and medium. We prove that the local variations of the population difference created by the electric field actually induce a nonlinear effect by coupling the fundamental of the field Fourier component to its harmonics. As a result, the multiscale averaging limit of the Maxwell-Bloch system is a new system of coupled nonlinear Schrodinger equations for the slowly varying envelope of the electric field. This system has one integrable limit when the electric field is circularly polarized: it is the scalar nonlinear Schrodinger equation. Moreover, it possesses three constants of the motion and a Hamiltonian formulation. Then we prove that our system is nonintegrable as it does not pass the Painleve test. Finally, a stability analysis shows that it is modulationally unstable and thus it propagates steady pulses of coherent light.